1. Field of the Invention
The present invention relates to a dental cement, particularly, to a dental glass ionomer cement. More particularly, the present invention is concerned with a dental glass ionomer cement composition designed to be simultaneously curable by polymerization.
2. Prior Art
Dental cements now available are of varied types and find wide applications, typically including a zinc phosphate cement making use of the reaction of zinc oxide with phosphoric acid, a polycarboxylate cement making use of the reaction of zinc oxide with a polycarboxylic acid, a zinc oxide eugenol cement making use of the reaction of zinc oxide with eugenol, a glass ionomer cement making use of the reaction of fluoroaluminosilicate glass powders with polycarboxylic acid and a resin cement making use of the polymerization of an acrylic base monomer.
As yet, any perfect and ideal dental cement is still unavailable, because such dental cements have their own merits and demerits. Referring to the demerits, the zinc phosphate cement is lacking in adhesion to tooth substance and tends to make phosphoric acid produce its own irritating action at the initial stage of setting; the polycarboxylate cement gives a hardened mass with a comparatively low final strength; the eugenol cement is not only of low strength and poor in intra-mouth durability so that its use is limited to temporal sealing and cementing, but also makes eugenol produce its own irritating action; and the resin cement is controversial in terms of biocompatibility.
Because of its merits such as improved biocompatibility, adhesion to tooth substance, satisfactory intra-oral durability and hardened mass having translucency and good appearance, on the other hand, the glass ionomer cement has now wide applications such as cementing of inlays, crowns and so on, filling of caries cavities, lining and preventive sealing of pits and fissures. However, the most serious problem with the glass ionomer cement is that when it is in touch with moisture such as saliva at the early stage of setting, its setting reaction is prevented from proceeding, ending up in deteriorations of the physical properties of the hardened mass.
The glass ionomer cement is obtained by the neutralizing reaction of a polycarboxylic acid (an acid) with fluoroaluminosilicate glass (a base) in the presence of water. Hence, that reaction is so readily affected by water that, upon coming into contact with water at the early stage of setting, its hardened mass is embrittled on its surface with a reduction in strength. At this time, the hardened mass clouds on its surface, offering an undesirable aesthetic problem. For improvements in glass ionomer cements, multiple attempts have been made up to date. For instance, Japanese Patent Publication Nos. 54-21858 and 57-2210 teach to make setting proceed rapidly by the addition of a chelating agent and a fluorocomplex, respectively. Never is any solution provided so far to this problem.